Add tests for variable init from global variable

Add tests for variable init from global variable

The existing OpVariable init tests were using
only constants as the initialization value. Now
also global scope variables are tested. There
are two variations for these global variables:
one where it's initialized from a constant and
one where the value is written in the code before
using the global as an initialization source.

Affects:

dEQP-VK.spirv_assembly.*.variable_init.*

New tests:

dEQP-VK.spirv_assembly.*.variable_init.*_from_*

Components: Vulkan

VK-GL-CTS issue: 1773

Change-Id: If19f8c3ec5b5da5626c41991add15eaa3183d77f